The present invention relates generally to radio communication systems and, in particular, to built-in antennas which can be incorporated into portable terminals and which allow the portable terminals to communicate within different frequency bands.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing the next generation technology, e.g., time division multiple access (TDMA) or code division multiple access (CDMA). Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure.
To provide an acceptable level of equipment compatibility, standards have been created in various regions of the world. For example, analog standards such as AMPS (Advanced Mobile Phone System), NMT (Nordic Mobile Telephone) and ETACS and digital standards such as D-AMPS (e.g., as specified in EIA/TIA-IS-54-B and IS-136) and GSM (Global System for Mobile Communications adopted by ETSI) have been promulgated to standardize design criteria for radio communication systems. Once created, these standards tend to be reused in the same or similar form, to specify additional systems. For example, in addition to the original GSM system, there also exists the DCS1800 (specified by ETSI) and PCS1900 (specified by JTC in J-STD-007), both of which are based on GSM.
However, the most recent evolution in cellular communication services involves the adoption of additional frequency bands for use in handling mobile communications, e.g., for Personal Communication Services (PCS) services. Taking the U.S. as an example, the Cellular hyperband is assigned two frequency bands (commonly referred to as the A frequency band and the B frequency band) for carrying and controlling communications in the 800 MHz region. The PCS hyperband, on the other hand, is specified in the United States to include six different frequency bands (A, B, C, D, E and F) in the 1900 MHz region. Thus, eight frequency bands are now available in any given service area of the U.S. to facilitate communication services. Certain standards have been approved for the PCS hyperband (e.g., PCS1900 (J-STD-007)), while others have been approved for the Cellular hyperband (e.g., D-AMPS (IS-136)).
Each one of the frequency bands specified for the Cellular and PCS hyperbands is allocated a plurality of traffic channels and at least one access or control channel. The control channel is used to control or supervise the operation of mobile stations by means of information transmitted to and received from the mobile stations. Such information may include incoming call signals, outgoing call signals, page signals, page response signals, location registration signals, voice channel assignments, maintenance instructions, hand-off, and cell selection or reselection instructions as a mobile station travels out of the radio coverage of one cell and into the radio coverage of another cell. The control and voice channels may operate using either analog modulation or digital modulation.
The signals transmitted by a base station in the downlink over the traffic and control channels are received by mobile or portable terminals, each of which have at least one antenna. Historically, portable terminals have employed a number of different types of antennas to receive and transmit signals over the air interface. For example, monopole antennas mounted perpendicularly to a conducting surface have been found to provide good radiation characteristics, desirable drive point impedances and relatively simple construction. Monopole antennas can be created in various physical forms. For example, rod or whip antennas have frequently been used in conjunction with portable terminals. For high frequency applications where an antenna""s length is to be minimized, another choice is the helical antenna.
It is commercially desirable to offer portable terminals which are capable of operating in widely different frequency bands, e.g., bands located in the 800 MHZ region and bands located in the 1900 MHZ region. Accordingly, antennas which provide adequate gain and bandwidth in both frequency bands will need to be employed in portable terminals. Several attempts have been made to create such dual-band antennas.
For example, U.S. Pat. No. 4,571,595 to Phillips et al. describes a dual-band antenna having a sawtooth-shaped conductor element. The dual-band antenna can be tuned to either of two closely spaced apart frequency bands (e.g, centered at 915 MHz and 960 MHz). This antenna design is, however, relatively inefficient since it is so physically close to the chassis of the mobile phone.
Japanese patent no. 6-37531 discloses a helix which contains an inner parasitic metal rod. In this patent, the antenna can be tuned to dual resonant frequencies by adjusting the position of the metal rod. Unfortunately, the bandwidth for this design is too narrow for use in cellular communications.
Dual-band, printed, monopole antennas are known in which dual resonance is achieve by the addition of a parasitic strip in close proximity to a printed monopole antenna. While such an antenna has enough bandwidth for cellular communications, it requires the addition of a parasitic strip. Moteco AB in Sweden has designed a coil matching dual-band whip antenna and coil antenna, in which dual resonance is achieved by adjusting the coil matching component (xc2xc xcex for 900 MHz and xc2xd xcex for 1800 MHz). This antenna has relatively good bandwidth and radiation performances and a length in the order of 40 mm. A non-uniform helical dual-band antenna which is relatively small in size is disclosed in copending, commonly assigned U.S. patent application Ser. No. 08/725,507, entitled xe2x80x9cMultiple Band Non-Uniform Helical Antennas.xe2x80x9d
Presently, antennas for radio communication devices, such as mobile phones, are mounted directly on the phone chassis. However, as the size and weight of portable terminals continue to decrease, the above-described antennas become less advantageous due to their size. Moreover, as the functionality of these future compact portable terminals increases, the need arises for a built-in miniature antenna which is capable of being resonant at multiple frequency bands.
Conventional built-in antennas currently in use in mobile phones include microstrip antennas and planar inverted-F antennas. Microstrip antennas are small in size and light in weight. The planar inverted-F antenna (PIFA) has already been implemented in a mobile phone handset, as described by K. Qassim, xe2x80x9cInverted-F Antenna for Portable Handsetsxe2x80x9d, IEE Colloqium on Microwave Filters and Antennas for Personal Communication Systems, pp.3/1-3/6, Feb. 1994, London, UK. More recently, Lai et al. have published a description of a meandering inverted-F antenna (WO 96/27219). This antenna has a size which is about 40% of that of the conventional PIFA antenna.
FIGS. 1A and 1B illustrate the conventional planar patch antenna compared to the meandering inverted-F antenna described in Lai et al. The conventional planar patch antenna of FIG. 1A has both a size and length equal to, for example, a quarter wavelength of the frequency to which the antenna is to be made resonant. The conventional planar patch antenna also has a width W. The meandering inverted-F antenna, illustrated in FIG. 1B, also has a length equal to a quarter wavelength of the resonant frequency and a width equal to W; however, the size of the meandering inverted-F antenna is reduced to about 40% of the size of the conventional planar patch antenna. This reduction in size is attributable to the antenna""s meandering shape.
However, as mobile phones become smaller and smaller, both conventional microstrip patch and PIFA antennas are still too large to fit future phone chassis. In copending, commonly assigned U.S. patent application Ser. No. 09/112,366, entitled xe2x80x9cMiniature Printed Spiral Antenna for Mobile Terminalsxe2x80x9d, a printed spiral built-in antenna with a matching post was proposed. The size of the antenna was reduced to 20-30% of the conventional PIFA antenna, which is less than {fraction (1/10)}th of a wavelength, in order to make it suitable for future mobile phones.
In addition to a reduced antenna size, next generation mobile phones will require the capability to tune to more than one frequency band for cellular, wireless local area network, GPS and diversity. In copending, commonly assigned U.S. patent application Ser. No. 09/112,152, entitled xe2x80x9cTwin Spiral Dual Band Antennaxe2x80x9d, a multiple band, built-in antenna was proposed which is suitable for future mobile phones. The built-in antenna comprises two spiral conductor arms which are of different lengths and capable of being tuned to different frequency bands. In this design, the bandwidth of the antenna is smaller because thin strip lines are used as radiators. In order to increase bandwidth of the antenna, a compensation method is used by introducing a resistor loading technique on the matching bridge. While this approach leads to a wider bandwidth, it also results in a loss of gain. This antenna is designed for use in two frequency bands.
In copending, commonly assigned U.S. patent application Ser. No. 09/212,259, entitled xe2x80x9cPrinted Multi-Band Patch Antennaxe2x80x9d, another new type of dual band patch antenna is disclosed. In contrast to the twin spiral dual band antenna which uses thin strip lines as radiators, the multi-band patch antenna uses patches with slot cutting. The patches are used as radiators and facilitate a wider bandwidth. The multi-band patch antenna is also designed for two frequency bands.
There continues, however, to exist a need for an efficient, miniature, built-in antenna which is capable of tuning to multiple frequency bands while simultaneously having a broad bandwidth in each of those multiple frequency bands. In addition, such antennas should be capable of tuning to a number of different frequency ranges within plural bands.
The present invention overcomes the above-identified deficiencies in the art by providing a miniature, semi built-in multi-band printed antenna which is suitable for use in future compact mobile terminals by facilitating operation in at least three frequency ranges. According to exemplary embodiments, a semi built-in multi-band printed antenna is provided which includes patch elements of different sizes and capable of being tuned to different frequency bands. On each patch element is formed a slot which divides the patch element into sub-parts. Each sub-part of a patch element is structured so as to be resonant at a frequency in the same frequency band to which the patch element is tuned. As a result, a high efficiency, broad band, multi-band, and surface mountable low profile antenna can be realized which can be used in three frequency ranges where one of the frequency ranges is outside one of the two frequency bands.